Various materials have heretofore been used as a packaging material for food products, beverages, medicines. For example, paper, synthetic resin sheets or films, laminates of paper and a synthetic resin sheet or film, etc. are used. Among them, packaging materials composed of a synthetic resin, particularly polyolefin sheets or films have excellent molding characteristics, high productivity, resistance to water and chemicals, etc. and therefore have been widely used in many fields of industry.
However, polyolefin sheets or films, which are highly resistant to water vapor transmission, have only a poor resistance to permeation of gases, for example, oxygen gas, and as a result even when products or articles such as food products, machines, etc. are packaged with a polyolefin sheet or film, the covered product or article suffers various problems such as oxidation or deterioration, or generation of rusts by the action of oxygen which permeates the sheet or film.
In order to solve these problems, it has conventionally been proposed to use a laminate material composed of a polyolefin sheet or film and a sheet or film of a resin which is resistant to gas permeation such as nylon, an ethylene-vinyl alcohol copolymer, etc.
However, such laminate materials releases a high exothermic energy upon burning, and they tend to give damages to incinerators when they are incinerated. Therefore, it is disadvantageous to employ such laminate materials in the field of industry where they are used in large amounts.
Moreover, containers molded from such laminate materials are insufficient in stiffness, heat resistance, etc. and therefore it is difficult to use such laminate materials as a forming material for producing containers for retort food products.
U.S. Pat. No. 4,572,854 discloses a multilayer film which comprises:
(a) an inner barrier layer D comprised of a saponified ethylenevinyl alcohol copolymer which contains about 15 to about 60 mole % ethylene units and which is saponified to at least approximately 90%, said inner barrier layer presenting a first surface and a second surface; PA1 (b) on at least one of said first and second surfaces, an adhesion-promoting layer C comprised of a modified polyolefin; and PA1 (c) adjacent to said adhesion-promoting layer, a layer B comprised of at least one polymer selected from the group consisting of a polypropylene homopolymer, a propylene copolymer, and said modified polyolefin, wherein all of the layers of said multilayer film are coextruded to form a multilayer composite, which is then biaxially oriented under substantially the same stretching conditions to form said multilayer film. PA1 (a) a gas permeation-resistant resin layer; PA1 (b) on each side of the gas permeation-resistant resin layer, a polyolefin based resin layer laminated on the gas permeation-resistant layer through an adhesive layer, said polyolefin based resin layer comprising a mixture of 30-80% by weight, based on the weight of the mixture, of at least one inorganic filler selected from the group consisting of talc and calcium carbonate and at least one resin selected from the group consisting of a polypropylene resin, a mixture of a polypropylene resin and an ethylene-propylene elastomer, and a mixture of a polypropylene resin and a polyethylene resin; PA1 (c) on the outer surface of each polylolefin based resin layer, a thermoplastic resin layer free from inorganic filler and laminated on the outer surface of the polyolefin based resin layer, said thermoplastic resin being selected from the group consisting of a polypropylene resin, and a mixture of a polypropylene resin and a polyethylene resin.
Said U.S. patent discloses a 4-unit, 7-layered film which comprises an inner barrier layer D, an adhesion-promoting layer C on the surface of the layer D, a polymer layer B comprised of a propylene homopolymer or propylene copolymer having a melting point of about 140.degree. C. or higher and a sealable layer A and further discloses that said polymer B may contain titanium dioxide, calcium carbonate and silicon dioxide as a pigment and said sealable layer A may contain a lubricant, an anti-blocking agent, etc.
However, the multilayer film disclosed in said U.S. patent is a film coextruded and then biaxially oriented and its thickness is very thin, 10-100 .mu.m. Therefore, this patent relates to a high strength film having gas barrier property and moisture resistance and it is almost impossible to form containers from this film by thermoforming due to orientation of the film. Even if this film is formed into a container, the obtained container is insufficient in strength and stiffness and cannot be used as a container.
Moreover, the polymer layer B of said multilayer film may contain calcium carbonate, etc., but this calcium carbonate is added as a pigment and its amount is 1-25% by weight and is not used for increasing mechanical strength of the multilayer film.
If calcium carbonate is contained in an amount much greater than 25% by weight in said polymer layer B and they are coextruded and biaxially oriented, the resulting multilayer film becomes porous and cannot be a gas-permeation resistant or moisture resistant film and furthermore cannot be thermoformed into a container.